In a consumable electrode DC arc welder, constant-voltage controlled power is supplied to a load which is constituted by a continuously fed, consumable welding rod electrode and a workpiece to be wrought by the welder. The welding rod electrode and the workpiece are short-circuited, and, then, the welding rod electrode is removed off the workpiece, whereby arc is generated between the electrode and the workpiece. The short-circuiting and the arcing are alternated for welding.
The consumable electrode DC arc welder also has a large DC reactor disposed in an output side of the welder. The DC reactor prevents current supplied to the load, i.e. load current, from rising abruptly, to thereby reduce spatters from a melted portion of the welding rod electrode. Spatters could be caused by excessive load current flowing during short-circuiting. A large DC reactor, however, hinders downsizing of the welder.
U.S. Pat. No. 5,457,300 assigned to the same assignee of the present application shows a consumable electrode DC arc welder in which load current flowing during short-circuiting is reduced to thereby reduce spatters, without causing any insufficient supply of load current during transition to arcing, whereby welding failure is prevented.
The DC arc welder according to U.S. Pat. No. 5,457,300 is described below with reference to FIG. 1. This U.S. Pat. No. 5,457,300 is also incorporated into the present application by reference. The welder has input terminals 1a, 1b and 1c to which AC power is supplied from e.g. a three-phase commercial AC power supply of a rated voltage of 200V. The AC power is rectified by an input rectifier 2 comprising e.g. bridge-connected diodes, and then smoothed by a smoothing capacitor 3 into DC power. The DC power is supplied to an inverter 4 comprising semiconductor switching elements, such as IGBTs and FETs, connected in bridge. The inverter 4 is pulse-width-modulation (PWM) controlled by an inverter control circuit 5 such that the switching elements are switched at a high frequency, thereby generating high-frequency power. The high-frequency power is lowered by an output transformer 6. The lowered high-frequency power is induced in a secondary winding 6s of the transformer 6.
The induced, lowered high-frequency power is rectified by an output rectifier 7 comprising e.g. bridge-connected diodes, and then supplied, via a smoothing and current-controlling DC reactor 8, between output terminals 9p and 9n. That is, conversion to DC power is provided also in an output side of the welder. The output terminal 9p is more positive than the output terminal 9n, and connected to a welding rod electrode 11 via an insulated cable 10p. The welding rod electrode 11 is wound in the form of a coil 12 and is continuously fed by a feeding device 13. The output terminal 9n is connected via an insulated cable 10n to a workpiece 14 to be wrought by the welder. The workpiece 14 and the welding rod electrode 11, constituting a load 15, are short-circuited, and, then, the welding rod electrode 11 is removed from the workpiece 14, so that arc is generated therebetween.
During welding operation, a voltage between the output terminals 9p and 9n, i.e. a load voltage V, changes as shown in FIG. 3(a), and current flowing through the load 15, i.e. load current I, changes as shown in FIG. 3(b). In FIG. 3, Ta denotes an arcing period, and Ts denotes a short-circuiting period.
The output transformer 6 has an auxiliary winding 6a in addition to the secondary winding 6s. An output voltage across the auxiliary winding 6a is rectified by an auxiliary rectifier 16. The auxiliary rectifier 16 provides a negative auxiliary signal Sv related to the load voltage V, to an inverting input terminal of an operational amplifier 18 through a resistor 17. A non-inverting input terminal of the operational amplifier 18 is grounded. A feedback resistor 19 is connected between the inverting input terminal and the output terminal of the operational amplifier 18.
The inverting input terminal of the operational amplifier 18 receives a reference signal, e.g. a reference voltage Sr, from a reference voltage source, e.g. a battery 21, through a resistor 20.
A current detector 22 detects the load current I and provides a load-current representative signal. The load-current representative signal is differentiated by a differentiation circuit 25 which comprises a resistor 23 and a capacitor 24 connected in series. The differentiation circuit 25 provides a differentiation output Idif, representing variations of the load current. The differentiation output Idif abruptly rises immediately after the beginning of the short-circuiting period and then gradually decreases, as shown in FIG. 3(c). It abruptly falls in the beginning portion of the arcing period and then gradually recovers. This differentiation output Idif is processed by an operational amplifier 26 with a feedback resistor 27. The operational amplifier 26 provides a negative signal Sx to the inverting input terminal of the operational amplifier 18 via a resistor 28. Thus, an input signal Sei applied to the inverting input terminal of the operational amplifier 18 is equal to (Sr-Sv-Sx). The operational amplifier 18, serving as an error amplifier, provides the inverter control circuit 5 with an error signal Se in accordance with the input signal Sei. The control circuit 5 controls the inverter 4 to change the output of the inverter, hence, the output of the welder, in such a manner that the signal Se can be cancelled, whereby the output of the welder is stabilized.
The input signal Sei applied to the inverting input terminal of the operational amplifier 18 abruptly falls immediately after the beginning of the short-circuiting period and then gradually recovers, as shown in FIG. 3(d). With a higher rate of increase of the load current I, the differentiation output Idif increases accordingly, which increases the signal Sx and, therefore, the input signal Sei decreases. This reduces the load voltage V and the load current I, thereby preventing spatters. The level of the input signal Sei rises in the beginning of the arcing period, and then decreases gradually.
If the workpiece 14 is a thick board, for example, the load current I has a higher rate of decrease and the differentiation output Idif has a reduced level, so that the correction signal Sx decreases. Thus, the input signal Sei has a higher level, so that the load voltage V and the load current I are increased. This increases supply of heat to the load, which can provide a strong weld of a large area. In contrast, if the workpiece 14 is a thin board, the load current I has a lower rate of decrease and the differentiation output Idif also has a lower level. Thus, the load current I is suppressed and the workpiece 14 can be prevented from being excessively melted.
A user may change the type, attitude and/or feeding speed of the welding rod electrode 11, depending on the material, size and the like of the workpiece, for example, and accordingly change the values of the resistors 23 and 28. This causes the rising and falling edges of the signal Sx to occur earlier as represented by a long dashed line in FIG. 4, or later as represented by an alternate long and short dashed line in FIG. 4, than the initial state as represented by a solid line in FIG. 4. In other words, the period of a cycle consisting of short-circuiting and arcing, and, therefore, the period of the load voltage V become longer or shorter. Therefore, for example, if a user initially sets the reference voltage Sr to such a value that the average load voltage of 20V is obtained, the average of the load voltage V applied during welding may change to 19V or 21V.
The object of the present invention is to provide a consumable electrode DC arc welder in which variations in period of the load voltage can be compensated for so that the load voltage can be maintained constant.